Anti-lock brake control system

ABSTRACT

A brake control system is described that prevents a wheel lockup condition by identifying the brake pressure that produces the maximum possible braking effort during each brake pressure application period and applying a predetermined fraction of the identified pressure after an incipient wheel lockup condition is sensed. The predetermined fraction provides for adaptive adjustment of the brake pressure if the identified brake pressure is in error until a pressure is applied that produces substantially the maximum possible braking effort.

This is a continuation of application Ser. No. 789,576 filed on Oct. 21,1985, now U.S. Pat. No. 4,664,453.

BACKGROUND OF THE INVENTION

This invention relates to an anti-lock control system for vehicle wheelbrakes.

When the brakes of a vehicle are applied, a braking force between thewheel and the road surface is generated that is dependent upon variousparameters including the road surface condition and the amount of slipbetween the wheel and the road surface. For a given road surface, theforce between the wheel and the road surface increases with increasingslip values to a peak force occurring at a critical wheel slip value. Asthe value of wheel slip increases beyond the critical slip value, theforce between the wheel and the road surface decreases. Stable brakingresults when the slip value is equal to or less than the critical slipvalue. However, when the slip value becomes greater than the criticalslip value, braking becomes unstable resulting in sudden wheel lockup,reduced vehicle stopping distance and a deterioration in the lateralstability of the vehicle.

Numerous wheel lock control systems have been proposed to prevent thewheels from locking up while being braked. Typically, these systemsprevent the wheels from locking by releasing the applied brake pressurewhen an incipient wheel lockup condition is sensed. One criteria that isused to sense an incipient wheel lockup condition is excessive wheeldeceleration. After release of the brake pressure, the wheeldeceleration ceases and the wheels then accelerate toward vehicle speed.When the wheel speed has substantially recovered, brake pressure isreapplied. One criteria that is typically used to sense recovery iswheel acceleration falling below a specified low value. Reapplication ofbrake pressure results in the wheel again approaching lockup and thecycle is repeated. This form of wheel lock control system results in arapid cycling of the brake pressure and the wheel slip value around thecritical slip value to prevent wheel lockup. Ten Hertz is a typicalcycling frequency.

SUMMARY OF THE INVENTION

As opposed to the foregoing systems for preventing wheel lockup duringbraking, the subject invention is directed toward a system whichidentifies the wheel brake pressure that results in the wheel slip beingat the critical slip value and which produces the maximum braking forcebetween the tire and the road surface. The brake pressure so identifiedis then applied to the wheel brake so as to substantially continuouslyestablish the critical slip value between the wheel and the road surfaceresulting in the maximum possible braking effort.

In general, the subject invention repeatedly calculates the brakingforce between the wheel and the road surface during braking from systemconstants and measured values and stores the brake pressurecorresponding in time to the peak calculated force. When an incipientwheel lockup is detected indicating that the critical wheel slip valveand therefore the peak braking force between the wheel and road surfacehas been exceeded, the stored brake pressure that produced the peakbraking force is reestablished to establish a braking condition n whichthe wheel slip is substantially at the critical slip value for theexisting road-tire interface condition.

In one aspect of this invention, the braking pressure reestablished toproduce the critical wheel slip value is automatically adjusted tocompensate for changing brake system parameters and for other errorsthat may exist in the calculation of the braking force to provide for asystem that self-adapts to those changes and errors.

DESCRIPTION OF THE DRAWINGS

The invention may be best understood by reference to the followingdescription of a preferred embodiment and the drawings in which:

FIG. 1 is a diagram illustrating the brake force coefficient between awheel and a road surface as a function of the percentage slip betweenthe wheel and road surface for two road surface conditions;

FIG. 2 is a general diagram of the braking system for controlling thebrakes in accord with the principles of this invention;

FIG. 3 is a longitudinal cross-sectional view of the actuator of FIG. 2for modulating the brake pressure to prevent wheel lockup;

FIG. 4 is a diagram of the electronic controller of FIG. 2 that isresponsive to brake system parameters for controlling the brake pressureto inhibit wheel lockup in accord with principles of this invention; and

FIGS. 5 thru 8 are diagrams illustrating the operation of the enginecontroller of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A wheel under the influence of braking has two major torques acting onit: brake torque and tire torque. Brake torque arises from theapplication of brake pressure through the brake mechanism and tiretorque is generated by the friction of the tire-road interface as wheelslip occurs.

Brake torque T_(b) is assumed to be proportional to brake pressure P_(b)with a known brake gain K_(b) and is defined by the expression

    T.sub.b =P.sub.b K.sub.b                                   (1)

Tire torque T_(t) is related to the brake force coefficient μ betweenthe tire and the road surface, the normal load N on the tire and thewheel rolling radius R and is defined by the expression

    T.sub.t =μNR.                                           (2)

For the free body consisting of the brake, wheel, and tire, the equationof motion is

    I.sub.w ω+T.sub.b -T.sub.t =0                        (3)

where I_(w) is the wheel moment of inertia and ω is the wheel angularacceleration. When the difference between the tire torque and the braketorque is positive, the wheel accelerates; and when negative, the wheeldecelerates.

Rearranging expression 3, tire torque T_(t) is defined as

    T.sub.t =I.sub.w ω+T.sub.b.                          (4)

As can be seen, the tire torque can be calculated from values that areeither known or can be measured. The wheel moment of inertia I_(w) andthe brake gain K_(b) are known values, the value of brake pressure P_(b)can be measured and ω can be determined by differentiating the value ofwheel speed which can be measured.

The brake friction coefficient term μ of the tire torque T_(t) is anonlinear function of the magnitude of slip between the wheel and theroad surface during braking and is dependent upon the road surfacecondition. FIG. 1 illustrates the brake friction coefficient μ as afunction of percentage-wheel slip for two road surface conditions. For agiven road surface, it can be seen that as wheel slip is increased inresponse to increased brake torque T_(b), the brake friction coefficientμ and therefore the tire torque T_(t) increases until a critical slipvalue at which the brake friction coefficient and the tire torque are ata maximum. A further increase in wheel slip results in a decrease in thebrake friction coefficient and tire torque. The maximum tire torqueresulting in a maximum braking effort for a given road surface isachieved when the brake torque T_(b) produces the critical wheel slipvalue. When the braking effort produces a wheel slip exceeding thecritical slip value, the braking operation becomes unstable andtypically results in sudden wheel lockup which in turn results inincreased stopping distance and a deterioration in the steering andlateral stability of the vehicle.

In general, the brake control system incorporating the principles ofthis invention identifies the value of the braking pressure P_(b) thatproduces the maximum tire torque T_(t). This is accomplished bycontinuously calculating the tire torque value T_(t) of equation (4)during braking. Any time the calculated value is larger than anypreviously calculated value, the value of the braking pressure P_(b) isstored so that the brake pressure producing the maximum tire torque isknown. When an incipient wheel lock is detected, the brake pressure isdumped to allow the wheel speed to recover and the brake pressure isthereafter reapplied to the stored value to establish a brakingcondition in which the wheel slip is substantially at the critical slipvalue for the existing road surface condition. This results insubstantially the maximum possible tire torque T_(t) and minimumstopping distance for the road surface condition.

If for some reason, such as a change in the brake system parameters overtime, there exists an error in the calculated tire torque which resultsin the storing and subsequent reapplication of an unstable brakepressure, the brake pressure is adaptively reduced until a stablepressure is identified that produces substantially the critical slipvalue and therefore the maximum braking effort. This is accomplished byestablishing the reapplied brake pressure at a predetermined fraction ofthe stored brake pressure P_(b) that corresponded in time to the maximumcalculated value of tire torque T_(t). If the resulting applied brakepressure results in a sensed incipient wheel lockup condition, the brakepressure next applied is again reduced by the friction. The repeatedreduction of the applied brake pressure by this predetermined fraction,such as 0.9, provides for self-adaptation to changing brake systemparameters including changes in the coefficient of friction of the brakelinings or for any errors in the coefficients utilized in thecalculation of tire torque.

A general overview of the wheel lock control system of this invention isillustrated in FIG. 2. The control of the brake of a single wheel isillustrated, it being understood that the control of the brakes of theremaining wheels of the vehicle are identical thereto. A standard wheelbrake 10 for a wheel 11 is actuated by controlled hydraulic pressurefrom one of two sources. The primary source is a motor driven actuator12 and the secondary source is a standard master cylinder 14 controlleddirectly by the vehicle brake pedal 16. A normally open electromagneticvalve 18 is energized when the actuator 12 is operative to control thehydraulic pressure to the brake 10 so as to decouple the master cylinder14 and brake pedal 16 from the hydraulic pressure output of the actuator12. This prevents pressure feedback to the vehicle operator while brakepressure is controlled by the actuator 12. When the electromagneticvalve 18 is deenergized, the hydraulic pressure to brake 10 may bemodulated directly by the brake pedal 16 and master cylinder 14.

The valve 18 is deenergized only during limited vehicle operatingconditions such as low vehicle speed or during failed conditions of theprimary hydraulic pressure source to permit brake pressure modulation bythe master cylinder 14. At all other times, the valve 18 is energized todecouple the master cylinder 14 from the braking system.

An electronic controller 20 is responsive to the outputs of a brakepedal force sensor 22 providing a signal that is a measure of theoperator applied brake pedal force F, a wheel speed sensor 24 thatprovides a signal that is a measure of wheel speed ω, and a pressuresensor 26 that provides a signal that is a measure of the hydraulicbrake pressure P_(b) applied to the brake 10 from the master cylinder 14or the actuator 12. The electronic controller 20 is responsive to thosesignals to (a) energize the valve 18 when the wheel speed ω exceeds avalue corresponding to a low vehicle speed such as 3 mph, (b) controlthe actuator 12 so as to apply a hydraulic pressure P_(b) to the brake10 that is proportional to the brake force F times a gain constant G forproviding power assist during normal braking conditions, and (c) limitthe pressure P_(b) applied to the brake 10 to a value that results inthe maximum possible tire torque T_(t) for the road surface condition toprevent wheel lockup and to provide for the shortest possible stoppingdistance, lateral vehicle stability and controllable vehicle steering.

Referring to FIG. 3, the actuator 12 in the preferred embodimentincludes a DC torque motor 28 whose output shaft drives an input gear 30which in turn rotatably drives an output gear 32. The drive member 34 ofa ball screw actuator is secured for rotation with the output gear 32.The drive member 34 engages and axially positions the driven member 36of the ball screw actuator. The driven member 36 drives a piston 38 tocontrol the hydraulic pressure output of the actuator 12. In summary,the torque output of the motor 28 is translated to a directly relatedhydraulic pressure P_(b) output of the actuator 12 that is applied tothe brake 10.

The actuator 12 may also take other forms. For example, it may take theform of a duty cycle modulated solenoid having an armature applying acontrolled force to the piston 38. Additionally, the motor 28 currentmay be used as a measure of the brake pressure P_(b) in place of thesensor 26 since the torque output of the motor 28 and therefore thehydraulic pressure P_(b) is related to the motor current.

As specifically illustrated in FIG. 4, the electronic controller 20 inthe present embodiment takes the form of a digital computer 40 and amotor control circuit 41. The digital computer is standard in form andincludes a central processing unit (CPU) which executes an operatingprogram permanently stored in a read-only memory (ROM) which also storestables and constants utilized in controlling the hydraulic pressureinput to the brake 10. Contained within the CPU are conventionalcounters, registers, accumulators, flag flip flops, etc. along with aclock which provides a high frequency clock signal.

The computer 40 also includes a random access memory (RAM) into whichdata may be temporarily stored and from which data may be read atvarious address locations determined in accord with the program storedin the ROM. A power control unit (PCU) receives battery voltage andprovides regulated power to the various operating circuits in theelectronic controller 20.

The computer 40 further includes an input/output circuit (I/O) that inturn includes a discrete output section controlled by the CPU to providea control signal to the valve 18. In controlling the brake 10, thecomputer outputs a digital signal to the motor control circuit 41 viathe I/O representing a desired value of the hydraulic brake pressure.The motor control circuit 41 converts the digital signal representingthe desired pressure to an analog signal which is compared with theactual measured value of the brake pressure P_(b). By standard closedloop adjustment that may include both proportional and integral terms,the motor 28 current is controlled so that the actual measured brakepressure P_(b) is made equal to the desired pressure.

The I/O also includes an input counter section which receives a pulseoutput from the wheel speed sensor 24 having a frequency representingwheel speed ω. Wheel speed ω is then determined by counting clock pulsesbetween wheel speed pulses.

An analog-to-digital unit (ADU) is included which provides for themeasurement of analog signals. The analog signals representingconditions upon which the hydraulic brake pressure to the brake 10 isbased are supplied to the ADU. In the present embodiment, those signalsinclude the brake pressure value P_(b) from the pressure sensor 26 andthe output of the brake pedal force sensor 22 providing a measure of thepedal force F. The analog signals are sampled and converted under thecontrol of the CPU and stored in ROM designated RAM memory locations.

The operation of the electronic controller 20 in controlling thehydraulic brake pressure P_(b) to the brake 10 in accord with theprinciples of this invention is illustrated in the FIGS. 5-8. Referringfirst to FIG. 5, when power is first applied to the system such as whenthe vehicle ignition switch is rotated to its "on" position, thecomputer program is initiated at point 42 and then proceeds to a step 44where the computer 40 provides for system initialization. For example,at this step initial values stored in the ROM are entered into ROMdesignated RAM memory locations and counters, flags and timers areinitialized.

After the initialization step 44, the program proceeds to a step 46where the program allows interrupts to occur and then to a backgroundloop 48 which is continuously repeated. This loop may include, forexample, diagnostic routines. In the preferred embodiment of thisinvention, an interrupt is provided by the CPU at 5 millisecondintervals during which the routines for establishing the hydraulic brakepressure P_(b) are executed.

Referring to FIG. 6, the five millisecond interrupt routine forcontrolling the vehicle brake 10 is illustrated. This routine is enteredat step 50 and proceeds to a step 52 where the last measured wheel speedω is saved and the new values of wheel speed ω, brake pedal force F andbrake line pressure P_(b) are read and stored in ROM designated RAMmemory locations. Next, the program proceeds to a step 54 where it isdetermined whether or not the operator is commanding brake application.The brakes are considered applied if the value of the brake pedal forceF is greater than zero. If the brakes are not applied, the programproceeds to a step 56 where a brake pressure command value P_(c) is setequal to zero. Also at this step, the speed of the vehicle asrepresented by the speed ω_(v) of a hypothetical unbraked wheel is setequal to the wheel speed measured at step 52. Since the brakes are notapplied, the wheel slip is substantially at zero so that the actual andhypothetical wheel speeds can be equated.

From step 56, the program proceeds to a step 58 where a D-flag is resetto condition the program to execute an identification routine(illustrated in FIG. 7) which identifies the brake pressure producingthe critical wheel slip value and therefore the maximum possible brakingeffort and which establishes the identified brake pressure following thesensing of an incipient wheel lockup condition. As will be described,the D-flag is set when an incipient wheel lockup condition is sensed tocondition the program to execute a dump routine (illustrated in FIG. 8)to release the brake pressure and allow the wheel speed to recover. Alsoat step 58, the maximum allowable brake line pressure P_(m) is set equalto a calibration constant K_(p) such as 1500 psi and a RAM memorylocation storing the value of the maximum calculated tire torque valueT_(tm) is set equal to zero. Thereafter, the program exits the 5millisecond interrupt routine and returns to the background loop 48 ofFIG. 5.

The foregoing steps 52 thru 58 are continuously repeated at 5millisecond intervals as long as the vehicle operator does not commandbrake application. However, when a force F is applied to the brakepedal, the program proceeds from step 54 to a series of steps thatprovide an estimation of the value of vehicle speed ω_(v) as representedby the speed of a hypothetical unbraked wheel. It is noted that theinitial value of ω_(v) was set equal to the actual wheel speed ω at step56 prior to operation of the brake pedal 16. This series of steps beginsat step 59 where the rate of change in wheel speed ω is determined fromthe old value of wheel speed saved at step 52 and the new value storedat step 52. The determined rate of change of wheel speed is thencompared with a constant deceleration of 1 g at step 60. The 1 gdeceleration value represents the maximum possible vehicle deceleration.When wheel deceleration is less than 1 g, it is assumed that the vehicleis decelerating at the same rate as the wheel 11. If, however, the wheeldeceleration exceeds 1 g, it is assumed that the vehicle decelerationremains at the maximum value of 1 g.

If the wheel deceleration is less than or equal to 1 g, the programproceeds from step 60 to a step 62 where ω is compared to zero. If thecomparison indicates wheel deceleration, the program proceeds to step 64where the rate of change of vehicle speed ω_(v) is set equal to theactual measured rate of change of wheel speed. If, however, thecomparison at step 62 indicates no change in wheel speed or wheelacceleration, the program proceeds to a step 66 where the rate of changeof vehicle speed ω_(v) is set equal to zero.

Returning to step 60, if it is determined that the wheel deceleration is1 g or greater, the program proceeds to a step 68 where is set equal tothe maximum possible vehicle deceleration of 1 g.

From the respective steps 64, 66 or 68, the program proceeds to a step70 where vehicle speed ω_(v) is estimated. This estimation is based onan initial value of vehicle speed ω_(v-1) determined during the previousexecution of the interrupt routine and the rate of change of vehiclespeed determined at step 64, 66 or 68 over the five millisecond intervalΔt between interrupt periods.

From step 70, the program proceeds to step 72 where the actual wheelspeed ω measured at step 52 is compared to the vehicle speed ω_(v)determined at step 70. If the wheel speed is equal to or greater thanthe vehicle speed (which cannot occur during braking of the wheel), thevalue of vehicle speed is corrected at step 74 by setting the vehiclespeed ω_(v) equal to wheel speed ω and the initial vehicle speed ω_(v-1)to be used at step 70 in the next execution of the interrupt routine isset equal to wheel speed ω. If at step 72 the wheel speed ω isdetermined to be less than the vehicle speed ω_(v), the program proceedsto a step 76 where the initial vehicle speed ω_(v-1) to be used at step70 during the next execution of the interrupt routine set equal to thevalue of vehicle speed determined at step 70.

Following step 74 or step 76, the program proceeds to a step 78 wherethe vehicle speed is compared to a calibration constant such as 3 mph.If the vehicle speed is less than 3 mph, the program proceeds to a step80 where the commanded brake line pressure P_(c) is set equal to thevalue of the brake pedal force F times a gain constant G for providingpower assisted braking. Thereafter, the program proceeds to a step 82where the valve 18 of FIG. 2 is deenergized and then to the step 58previously described.

If the vehicle speed is greater than 3 mph, the program proceeds fromstep 78 to step 84 where the valve 18 is energized to decouple themaster cylinder 14 from the actuator 12. Brake application is thereafterprovided solely via the actuator 12 as controlled by the electroniccontroller 20. From step 84, the program proceeds to a step 86 where thestate of the D-flag is sampled. If the D-flag is reset to condition theprogram to execute the identify routine, the program proceeds to a step88 where the identify routine is executed.

If step 86 determines that the D-flag is set, the program is conditionedto execute a dump routine, and the program proceeds to a step 90 wherethe dump routine is executed. During this routine, the pressure to thebrake 10 is released to allow the speed of the wheel 11 to recover froman incipient lockup condition. Following the steps 88 or 90, the programexits the 5 millisecond interrupt routine of FIG. 6 and returns to thebackground loop 48 of FIG. 5.

Referring to FIG. 7, the identify routine 88 of FIG. 6 is illustrated.This routine (A) provides for power assisted braking, (B) identifies thebrake line pressure producing the critical wheel slip corresponding tothe maximum possible braking force between the tire and the roadsurface, (C) senses an incipient wheel lockup condition and conditionsthe program to execute the dump routine to allow wheel recovery from thelockup condition, (D) reestablishes the brake line pressure to theidentified pressure producing the critical slip value and (E) adaptivelyreduces the brake line pressure until a stable braking pressure isidentified if the pressure originally identified results in unstablebraking due to system parameter changes.

The identify routine is entered at point 92 and proceeds to a step 94where the value of the tire torque T_(t) is calculated in accord withthe equation (4) from the wheel deceleration ω determined at step 59,the brake line pressure P_(b) measured at step 52 and the known valuesof wheel inertia I_(w) and brake gain K_(b) From step 94, the programproceeds to steps 96 and 98 that function to identify the brake pressureproducing the maximum value of tire torque. At step 96, the tire torqueT_(t) calculated at step 94 is compared with the largest previouslycalculated value T_(tm) stored in memory. If the value calculated atstep 94 is greater than the stored value T_(tm), the program proceeds toa step 98 where the stored value T_(tm) is set equal to the larger valuecalculated at step 94. Also at this step, a stored value of brake linepressure P_(bm) is set equal to the brake line pressure P_(b) measuredat step 52. Therefore, the stored value P_(bm) represents the brake linepressure corresponding in time to the maximum calculated value of tiretorque stored at step 98.

The foregoing sequence of steps 96 and 98 are repeated with eachexecution of the identify routine as long as the tire torque isincreasing so that the brake line pressure resulting in the maximumcalculated value of tire torque is always known. If step 96 shoulddetermine that the calculated value of tire torque T_(t) is less thanthe stored maximum calculated value T_(tm), step 98 is bypassed. Thiswill occur when the brake pressure P_(b) results in a wheel slip thatexceeds the critical value which in turn results in a decrease in thetire torque. The stored value of brake pressure P_(bm) then representsthe brake line pressure establishing the critical wheel slip value andtherefore the maximum braking effort. As will be described, this storedbrake line pressure P_(bm) is utilized after an incipient wheel lockcondition is detected to reestablish a braking condition that producessubstantially the critical wheel slip value.

The program next determines whether or not an incipient wheel lockcondition exists as represented by excessive wheel deceleration orexcessive wheel slip. At step 100, the rate of change in wheel speeddetermined at step 59 is compare with a deceleration reference valuesuch as 10 g which, if exceeded, indicates that braking has becomeunstable and an incipient wheel lockup condition exists. If step 100does not detect an incipient wheel lockup condition, the programproceeds to step 102 where the ratio ω/ω_(v) is compared with areference value S_(m) which represents a wheel slip value that exceedsthe largest possible critical wheel slip value for any road surfacecondition. A ratio less than S_(m) indicates that braking has becomeunstable and an incipient wheel lockup condition exists.

If neither of the steps 100 and 102 detects an incipient wheel lockupcondition, the program proceeds to a step 104 where the value of theoperator requested brake pressure that is equal to the applied pedalforce F times the power assist gain factor G is compared with a maximumallowable brake line pressure P_(m). If the product is less than themaximum value, the program proceeds to a step 106 where the commandedbrake pressure value P_(c) is adjusted toward the operator requestedpressure in accord with a first order lag filter equation to providepower assisted braking. Thereafter, the program exits the identityroutine and returns to the background loop 48.

If at step 104 it is determined that the operator requested brakepressure is greater than the maximum allowable pressure P_(m), theprogram proceeds to a pressure ramp routine where, through repeatedexecutions of the identify routine, the maximum allowable brake pressureP_(m) and the commanded brake line pressure P_(c) are ramped up untilstep 104 detects that the maximum allowable brake pressure P_(m) hasbecome greater than the operator requested pressure or, if the operatorrequested brake pressure results in an unstable braking condition, untilthe commanded brake pressure results in an incipient wheel lockupcondition at which time the brake pressure establishing the criticalslip value has been identified by the steps 96 and 98. As will bedescribed, the brake pressure so identified is then used to reestablishthe commanded brake pressure after the wheel recovers from the incipientlockup condition. The result of the ramping of the brake pressure is aperiodic low frequency (such as 2 Hz) reidentification of the brakepressure producing the critical wheel slip value. This enables thesystem to adapt to increasing values of the brake friction coefficientresulting from changes in the tire-road surface interface.

The routine for ramping the brake pressure RAM timing register iscompared to zero. The initial value of time t₁ establishes a delay inthe ramping of the commanded brake pressure P_(c). Thereafter, the timet₁ functions in establishing the ramp rate. If the time t₁ is greaterthan zero, the program proceeds to a step 110 where the time t₁ isdecremented. Thereafter, at step 112, the program proceeds to adjust thecommanded brake pressure P_(c) toward a predetermined fraction of themaximum allowable brake pressure P_(m) in accord with a first order lagfilter equation. By setting the maximum allowable brake pressure P_(m)to the stored pressure P_(bm) after an incipient wheel lockup conditionis sensed (as will be described), the commanded pressure established atstep 112 will be the predetermined fraction of the pressure producingthe critical wheel slip. In one embodiment, the predetermined fractionis 0.9 so that the resultant brake pressure produces substantially thecritical wheel slip value.

As long as an incipient wheel lock condition is not detected and theoperator requested brake pressure is greater than the maximum allowablebrake line pressure P_(m), the steps 108 thru 112 are repeated at thefive millisecond interrupt interval until t₁ has been decremented tozero. After t₁ has been decremented to zero, the program proceeds fromstep 108 to step 114 where the time t₂ in a RAM timing register iscompared to zero. If the time t₂ is greater than zero, the programproceeds to a step 116 where the time t₂ is decremented.

Following step 116 or step 114, the program proceeds to a step 118 wherethe maximum allowable brake pressure P_(m) is incremented and the time tis set equal to K_(n) (t₂ +1). Thereafter, the steps 114 thru 118 willbe by passed upon repeated executions of the identify routine until t₁is again decremented to zero. From this it can be seen that the maximumallowable brake pressure P_(m) is periodically incremented at intervalsdetermined by K_(n) and t₂. When t₂ is decremented to zero, the maximumallowable brake line pressure P_(m) is incremented with each K_(n)executions of the identify routine.

Following step 118, the program proceeds to step 112 where the commandedbrake line pressure P_(c) is again set as previously described. Repeatedexecutions of the foregoing steps function to increase the commandedbrake pressure P_(c) exponentially. This increase will be continueduntil (A) an incipient wheel lock condition is forced so as to force areidentification of the brake pressure producing the critical slip valuevia the steps 96 and 98 or (B) the operator requested brake pressurebecomes less than the maximum allowable pressure P_(m).

If the commanded brake pressure P_(c) is increased to a point resultingin the wheel slip value becoming greater than the critical slip value,the wheels then quickly approach a lockup condition. This incipientwheel lock condition is detected as previously described at step 100 orstep 102. When the incipient wheel lockup condition is detected, thebrake line pressure P_(bm) in memory at that time is the brake linepressure producing the critical wheel slip value and therefore themaximum possible tire torque.

After a wheel lockup condition has been sensed, the program proceeds toa step 120 where the time t₂ is compared with a constant t_(k1). As willbe seen, these two values will be equal only if a wheel lockup conditionis sensed within a predetermined time t_(k2) (such as 500 ms) after thebrake pressure is reestablished after recovery from an incipient wheellockup condition. A wheel lockup occurring within this period afterreapplication of the brake pressure implies the application of anunstable brake pressure producing an incipient wheel lockup condition.If this condition exists, the program proceeds to a step 122 where thebrake pressure P_(bm), stored at step 98 and identified as the pressureestablishing the critical wheel slip value, is compared with thecommanded brake pressure P_(c) which resulted in the incipient wheellockup condition. If greater, the program proceeds to a step 124 wherethe stored value of P_(bm) is corrected to the commanded pressure P_(c).This condition represents an error in the calculation of the tire torqueeither through changes in the brake line coefficients or errors invarious constants used in the determination of the calculation of thetire torque T_(t). Since the brake line pressure producing the criticalslip value can never be greater than the commanded brake line pressureP_(c) that resulted in an incipient wheel lock condition, the value ofP_(bm) is reduced to the value of P_(c) causing the incipient wheel lockcondition.

From step 120 if the time t₂ is not equal to t_(k1), from step 122 ifP_(bm) is less than P_(c), or from step 124, the program proceeds to astep 126 where the D-flag is set to condition the program to execute thedump routine and certain initial conditions for reapplication of brakepressure are established. The initial conditions include setting themaximum allowable brake pressure P_(m) equal to the stored value ofbrake pressure P_(bm) (the brake pressure identified as producing thecritical wheel slip value), setting the time t₁ equal to the constantt_(k2) and setting the time t₂ equal to the constant t_(k1). The programnext proceeds to a step 128 where the dump routine is executed.Thereafter, during executions of the 5 ms interrupt routine of FIG. 6,the identify routine is bypassed via the step 86 and the dump routine 90is executed until the D-flag is again reset.

The dump routine executed at step 128 of the identify routine of FIG. 7and at step 90 of the interrupt routine of FIG. 6 is illustrated in FIG.8. This routine is entered at point 130 and proceeds to step 132 wherewheel slip represented by the ratio of wheel speed ω to the speed ω_(v)of the hypothetical unbraked wheel is compared to a constant S_(k)representing wheel speed approaching vehicle speed. S_(k) may be, forexample, 0.9 representing a wheel slip or 10 percent. If the ratio isless than S_(k), the program proceeds to a step 134 where the commandedbrake pressure P_(c) is set to zero to allow the wheel speed to recoverfrom the incipient wheel lockup and toward vehicle speed. When step 132detects wheel speed recovery, the program proceeds from step 132 to astep 136 where the D-flag is reset to condition the program to executethe identify routine of FIG. 7. Also at this step, the maximum value ofcalculated tire torque T_(tm) is set to zero so that the identifyroutine is conditioned to reidentify the brake pressure establishing thecritical wheel slip value. The program then exits the dump routine ofFIG. 8 and returns to the background loop 48.

During the following executions of the 5 millisecond interrupt routineof FIG. 6, the program executes the identify routine at step 88 untilthe D-flag is again set at step 126 after an incipient wheel lockupcondition is sensed.

A brief summary of operation will now be described. At step 58 prior tothe operator applying the vehicle brakes and at step 136 prior to brakepressure being reapplied after being released by the dump routine ofFIG. 8, the stored maximum value T_(tm) of calculated tire torque is setto zero so that prior to each application of brake pressure, theidentify routine is conditioned to identify the brake pressurecorresponding in time to the maximum calculated tire torque T_(tm).

First it is assumed that there are no errors in the coefficients andvalues used in the calculation of tire torque T_(t) in step 94 and thatthe tire-road surface interface conditions do not change during braking.As the commanded brake pressure P_(c) is increased via steps 104 and 106or ramped up via the steps 104 and steps 108 through 118, the storedbrake pressure value P_(bm) is continually updated with each increasingcalculated value of tire torque.

When an incipient wheel lockup condition is sensed at step 100 or 102,the stored value of P_(bm) is the brake pressure that resulted in thewheel slip being at the critical value and which produced the maximumpossible braking effort for the existing tire-road interface condition.The stored value of P_(bm) is unaffected by steps 120 through 124 sincethere were no errors in the calculated value of tire torque so that thevalue of P_(bm) could never exceed the commanded brake pressure P_(c)The maximum allowable brake pressure P_(m) is then set equal to thevalue of P_(bm) at step 126.

The D-flag is then set at step 126 after which the dump routine of FIG.8 is repeated during each 5 ms interrupt to release the brake pressureto allow the wheel speed to recover. When the wheel speed has recovered,the D-flag is reset at step 136 so that the identify routine is thenexecuted during each 5 ms interrupt. When executed, the identify routinereapplies the brake pressure at step 112 to the predetermined fractionof the maximum allowable brake pressure P_(m). Recalling that P_(m) wasset equal to the value P_(bm) that established the critical wheel slipvalue, the brake pressure reapplied is the predetermined fraction of thepressure establishing the critical wheel slip value. Since the fraction,which provides the adaptive feature of this invention, is typicallylarge, such as 0.9, the brake pressure reapplied results in stablebraking while at a wheel slip value substantially equal to the criticalwheel slip value. After the time t_(k2) (the initial value of t₁ set atstep 126) which is typically 500 ms, the brake pressure is slowly rampeduntil an incipient wheel lock condition is again detected. While thebrake pressure is being reapplied and thereafter ramped, the steps 96and 98 are functioning to reidentify and store the brake pressureproducing the critical wheel slip value.

The foregoing cycle is continually repeated as long as the operatorrequested pressure is greater than the maximum allowable pressure P_(m).The cycle time is slow (less than 2 Hz) so that the wheel slip issubstantially continuously controlled at the critical value.

If the road surface friction coefficient should increase while the brakepressure is being limited to prevent wheel lockup, the systemautomatically adapts to the change via the brake pressure ramp-upfunction provided by the steps 108 through 118. By ramping the brakepressure, the system is caused to reidentify the brake pressureproducing the critical wheel slip value and in so doing, adapts to thechange in the tire-road surface interface.

If the road surface friction coefficient should decrease while the brakepressure is being limited to prevent wheel lockup, the commanded brakepressure P_(c) becomes excessive resulting in the wheel deceleratingtoward lockup. This incipient lockup condition is sensed at step 100 or102 and the brake pressure released and subsequently reapplied aspreviously described. During reapplication of the brake pressure, steps96 and 98 will reidentify the brake pressure producing the maximumcalculated braking effort for the new tire-road surface interfacecondition as the wheel decelerates again toward lockup. This pressure(reduced by the fraction at step 112) is then applied as previouslydescribed after the brake pressure is next released in response to thesensed incipient wheel lockup.

The operation of the system in adapting to error in the identificationof the brake pressure establishing the critical wheel slip will now bedescribed. Over a period of time, changes may occur in the brake systemparameters that could result in an unstable brake pressure beingidentified as the pressure establishing the critical wheel slip value.When this pressure is then applied after the wheel has recovered, thewheel immediately begins to decelerate toward lockup. Withoutcorrection, the brake pressure would rapidly cycle between pressurerelease and pressure apply. One of the brake system parameters that maychange over time to produce the above result is the brake gain K_(b)used in calculating brake torque T_(b) in equation (4) above. The gainis affected, for example, by changes in the coefficient of friction ofthe brake linings in the brake 10.

By setting the maximum brake pressure P_(m) equal to the stored brakepressure value P_(bm) at step 126 and, after wheel speed recovery, bysetting the commanded brake pressure P_(c) to the predetermined fractionof P_(m), the commanded brake pressure P_(c) will be repeatedly steppeddown by the fraction as the system cycles between brake pressure releaseand pressure apply until the commanded pressure P_(c) becomes equal toor less than the value producing the critical wheel slip value. Forexample, if the predetermined fraction of the value of P_(bm) stored bythe identify routine is an unstable brake pressure due to errors such asabove described, the resulting brake command pressure P_(c) establishedat step 112 will cause an incipient wheel lockup condition at leastwithin the delay period t_(k2) previously described. The stored valueP_(bm) of the brake pressure at the time the incipient wheel lockup issensed cannot be greater than the command pressure P_(c). Therefore, themaximum allowable brake pressure P_(m) set at step 126 is less than theprior maximum allowable pressure by at least the amount determined bythe predetermined fraction. When the wheel speed has recovered via thedump routine, the new commanded brake pressure P_(c) is set to thepredetermined fraction of the new value of P_(m). This new commandedpressure value P_(c) is therefore at least less than the value of P_(c)in the prior cycle by an amount determined by the pressure fraction. Ifthe new command pressure P_(c) is still an unstable pressure, the cyclewill be repeated until the command pressure has been repeatedly reducedby the fraction to a stable pressure that establishes substantially thecritical wheel slip value.

The steps 122 and 124 further function to reduce the brake pressurereapplied in any cycle where the relationship between the storedpressure P_(bm) and the command pressure P_(c) represents an unstablebrake pressure condition. Once a stable pressure has been identified bythe above sequence, the system will not cycle again until expiration ofthe time period t_(k2) via steps 108 and 110 and thereafter the commandpressure P_(c) is ramped to an unstable pressure by the steps 108through 118 to force a reidentification of the pressure producing thecritical wheel slip value.

The foregoing description of a preferred embodiment for the purpose ofexplaining the principles of this invention is not to be considered aslimiting or restricting the invention since many modifications may bemade by the exercise of skill in the art without departing from thescope of the invention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A wheel lock controlsystem for limiting the brake pressure applied to the brakes of avehicle wheel traveling over a road surface, the system comprising:meansfor determining the tire torque tending to accelerate the wheel duringthe application of brake pressure; means for storing a valverepresentative of the brake pressure corresponding in time to themaximum determined tire torque; means for detecting an incipient wheellockup condition resulting from the valve of slip between the wheel andthe road surface exceeding a critical slip value; and means forestablishing the brake pressure following a detected incipient wheellockup condition at a value having a predetermined relationship to thebrake pressure represented by the stored value.
 2. A wheel lock controlsystem for limiting the brake pressure applied to the brakes of avehicle wheel traveling over a road surface, the system comprising:meansfor determining the tire torque tending to accelerate the wheel duringthe application of brake pressure; means for storing a valuerepresenting the brake pressure corresponding in time to the maximumdetermined tire torque during periods of application of brake pressure;means for detecting an incipient wheel lockup condition resulting fromthe value of slip between the wheel and the road surface exceeding acritical slip value; means for lowering the brake pressure in responseto a detected incipient wheel lockup condition to allow wheel speedrecovery; and means for reestablishing brake pressure following wheelspeed recovery to a value having a predetermined relationship to thebrake pressure represented by the stored value.
 3. The method oflimiting the brake pressure applied to the brakes of a vehicle wheeltraveling over a roadway comprising the steps of:determining the tiretorque tending to accelerate the wheel during the application of brakepressure; storing a value representing the brake pressure correspondingin time to the maximum determined tire torque following each applicationof brake pressure; detecting an incipient wheel lockup conditionresulting from the value of slip between the wheel and the road surfaceexceeding a critical slip value; and reapplying the brake pressurefollowing a detected incipient wheel lockup condition to a value that isa predetermined relationship to the brake pressure represented by thelast stored value.
 4. The method of limiting the brake pressure appliedto the brakes of a vehicle wheel traveling over a roadway comprising thesteps of:determining the tire torque tending to accelerate the wheelduring the application of brake pressure; storing a value representingthe brake pressure corresponding in time to the maximum determined tiretorque following each application of brake pressure; detecting anincipient wheel lockup condition resulting from the value of slipbetween the wheel and the road surface exceeding a critical slip value;lowering the brake pressure in response to a detected incipient wheellockup condition to allow wheel speed recovery; and reapplying brakepressure following wheel speed recovery to a value that is predeterminedrelationship to the brake pressure represented by the last stored value.5. A system for controlling the brake applied to the brakes of a wheeltraveling over a road surface to limit the wheel slip between the wheeland the road surface, the system comprising, in combination:means fordetermining the value of tire torque resulting from the tire and roadinterface; means for storing a value representative of the brakepressure corresponding in time to the maximum determined value of tiretorque; means for determining when the value of slip between the tireand road surface exceeds a critical slip value; and means responsive tothe determination of a slip value exceeding the critical slip value forreestablishing the brake pressure to a value related to the stored valueto establish a wheel slip value producing substantially the peak valueof tire torque.
 6. A system for controlling the brake pressure appliedto the brakes of a wheel traveling over a road surface to limit thewheel slip between the wheel and the road surface, the systemcomprising, in combination:means for determining the value of tiretorque resulting from the tire and road interface; and means forcyclically (A) storing a value representative of the brake pressurecorresponding in time to the maximum determined value of tire torque,(B) determining when the value of slip between the tire and road surfaceexceeds a critical slip value, (C) reestablishing the brake pressure toa value related to the stored value to establish a wheel slip valueproducing substantially the peak value of tire torque and (D) rampingthe brake pressure in direction tending to increase the value of slipbetween the tire and road surface, whereby the slip between the tire androad surface is maintained substantially at the value resulting in thepeak tire torque value.
 7. A method for controlling the brake pressureapplied to the brakes of a wheel traveling over a road surface to limitthe wheel slip between the wheel and the road surface, the methodcomprising:determining the value of tire torque resulting from the tireand road interface; storing a value representative of the brake pressurecorresponding in time to the maximum determined value of tire torque;determining when the value of slip between the tire and road surfaceexceeds a critical slip value; and reestablishing the brake pressurefollowing the determination of the value of slip exceeding a criticalslip value to a value related to the stored value to establish a wheelslip value producing substantially the peak value of tire torque. .Iadd.8. A method of limiting the brake pressure applied to the brakes of avehicle wheel traveling over a roadway, the method comprising the stepsof:detecting an incipient wheel lockup condition; storing a valuerepresenting the brake pressure corresponding to a predetermined brakingcondition between the wheel and the roadway; lowering the brake pressurein response to a detected incipient wheel lockup condition to allowwheel speed recovery; and reapplying the brake pressure following wheelspeed recovery to a value that is a predetermined fraction of thepressure represented by the stored value. .Iaddend. .Iadd.
 9. A methodof limiting the brake pressure applied to the brakes of a vehicle wheeltraveling over a roadway, the method comprising the steps of:detectingan incipient wheel lockup condition; storing a value representing thebrake pressure substantially at the time an incipient wheel lockupcondition is detected; lowering the brake pressure in response to adetected incipient wheel lockup condition to allow wheel speed recovery;reapplying the brake pressure following wheel speed recovery to a valuethat is a predetermined fraction of the pressure represented by thestored value; and ramping the reapplied brake pressure until anincipient wheel lockup condition is again detected. .Iaddend. .Iadd. 10.A method of limiting the brake pressure applied to the brakes of avehicle wheel traveling over a roadway, the method comprising the stepsof:detecting an incipient wheel lockup condition of the wheel resultingfrom slip between the wheel and the road surface exceeding a criticalslip value; storing a value representing the brake pressure establishingthe critical slip between the wheel and the road surface; lowering thebrake pressure in response to a detected incipient wheel lockupcondition to allow wheel speed recovery; reapplying the brake pressurefollowing wheel speed recovery to a value that is a predeterminedfraction of the pressure represented by the stored value; and rampingthe reapplied brake pressure until an incipient wheel lockup conditionis again detected. .Iaddend. .Iadd.
 11. A wheel lock control system forlimiting the brake pressure applied to the brakes of a vehicle wheeltraveling over a road surface, the system comprising:means for detectingan incipient wheel lockup condition; means for storing a valuerepresenting the brake pressure corresponding to a predetermined brakingcondition between the wheel and the road surface; means for lowering thebrake pressure in response to a detected incipient wheel lockupcondition to allow wheel speed recovery; and means for reestablishingbrake pressure following wheel speed recovery to a value that ispredetermined fraction of the pressure represented by the stored value..Iaddend.